A polyolefin microporous film has been widely used as a separator for various batteries such as primary and secondary lithium batteries, a lithium-polymer battery, a nickel-hydrogen battery, a nickel-cadmium battery, a nickel-zinc battery, and the like, a filter, a filter for separation, a separation membrane for micro-filtration, or the like. In the case in which the polyolefin microporous film is used as the separator for various batteries, performance of the separator is an important factor in properties, productivity, and stability of a battery. Therefore, the separator should have suitable mechanical properties, heat resistance, permeability, dimensional stability, and shutdown characteristics, and the like.
As a method of manufacturing a microporous film from polyolefin, there are a method of manufacturing a thin fiber using polyolefin to form a microporous film in a non-woven fabric form, a dry method of manufacturing a thick polyolefin film and then stretching the thick polyolefin film to form fine pores, and a wet method of kneading polyolefin with diluent to form a single phase and extracting a diluent portion after phase separation to form pores in a polyolefin film. Among them, in the wet method, a thin film having a thin and uniform thickness may be manufactured, and excellent physical properties may be implemented as compared to the other two methods, such that the wet method is suitable for a method of manufacturing a separator for a secondary battery.
The polyolefin microporous film is mainly used in a multilayer form for mechanical properties. For example, a polypropylene/polyethylene/polypropylene triple-layered separator has been used. This triple-layered separator is manufactured by separately manufacturing precursor films of respective layers, forming a multilayer film by a lamination method, and then stretching the multilayer film. Since in the separator manufactured as described above, each of the layers are not stretched under suitable conditions, such that there is a limitation in increasing permeability as compared to a single layer product, and it is difficult to thin the separator. Further, in the case of forming a multilayer separator by the lamination method, a separate lamination process should be performed, such that a process becomes complicated, and production cost is increased.
Therefore, a co-extrusion method of simultaneously extruding molten resins of the respective precursor films using two or more extruders to laminating a plurality of films in a molten state is used. This method may omit the lamination process, such that a process may be shortened. In addition, interlayer peel strength may be increased.
However, at the time of manufacturing a multilayer separator such as the triple-layered separator using the co-extrusion method, it is difficult to manufacture a precursor film having a high degree of alignment. Therefore, at the time of performing a stretching process, a stretching effect may be deteriorated, such that permeability of a final separator may be decreased, and                1 properties may be deteriorated.        